The interior of metal fluid-conducting conduits, such as those found in gas and oil pipelines, must be cleaned periodically to eliminate the accumulations which build up over time. Typically, the interior wall surface of a pipe is cleaned by use of a blast of sand which is pressurized to impinge the wall surface. To effect uniform cleaning, sandblasting equipment for pipes includes a rotating nozzle member that is moved along the length of a pipe undergoing cleaning. U.S. Pat. Nos. 3,137,974 to Kirkland, 3,902,276 to Jarvis and 4,314,427 to Stolz teach spin-blast tools. Kirkland, for example, discloses a spin-blast tool of a type having a hollow tubular handle connected at one end to a blast-slurry supply pipe and at the opposite end to a spinning jet head. The blast-slurry supply pipe carries an abrasive material such as sand in a sand-air mixture at approximately 110 pounds of pressure. Under this pressure, jet head rotation will exceed 1,000 revolutions per minute, causing severe vibration and, ultimately, damage to the tool.
Centrifugal brakes have been provided to slow the rotation of the jet head of a spin-blast tool. However, it is not uncommon for brake linings to be in need of replacement after only four hours of use. Worn or defective brake linings will result in excessive rotational speed of the jet head. As a result, bearing members and other parts within the spin-blast tool wear prematurely.
The difficulties in providing a properly operating brake mechanism in a spin-blast tool result from the rigors associated with a tool which projects a pressurized sand-air mixture into a confined space. Moreover, it is important that a brake mechanism of a spin-blast tool restrain the rotational speed of the jet head but not so much as to diminish performance of the tool.
An object of the present invention is to provide a brake mechanism which is adjustable and which withstands the rigors of a spin-blast tool.